Ink jet printing is a non-impact printing process in which a digital signal produces droplets of ink on media substrates such as paper or transparency films. Ink jet is widely used in industrial and office applications. Thermal or bubble jet drop-on-demand ink jet printers are the generally accepted medium price printers for personal computers. These printers have been widely accepted due to their high print quality, low cost, relatively quiet operation, environmental safety and color graphic capability.
High dot gain inks are desirable for ink jet printing because they give a large dot size for a given drop volume. The major advantage is that lower drop volumes can be used for a given printing application, resulting in lower cost per copy, reduced paper cockle and reduced amount of ink on transparency film. Most high dot gain inks have low surface tension which causes the ink to spread across the surface of the top plate of the pen cartridge, or so-called "puddling." The ink drops must fire through the puddles, which results in misdirected drops and streaking of images, and lowers printhead reliability.
A critical requirement for an ink jet ink is the ability to remain in a fluid condition in a pen opening on exposure to air, so-called "decap" conditions. This allows a pen to function after a period of non-use ("short-term decap") or during operation of infrequently utilized nozzles ("long-term decap"). A major concern with all ink jet printers is pluggage of nozzles during operation and between operations. This is caused by evaporation of an organic solvent or water from the surface of the nozzle. In dye containing inks, this can cause crystallization or precipitation of the dye or other solids, commonly referred to as "crusting".
Initial evaporation generally causes an increase in viscosity which affects the ability of the nozzle to fire a drop of ink since ink jet pens are designed to operate within specific viscosity ranges. The inception of crusting may cause distortion of the image, which may appear as a drop of ink which is displaced from its intended position or a splitting of the ink drop into two or more droplets displaced from the intended target position. On occasion, the drop may reach its intended position but at a lower drop volume, producing a lower optical density image. Ultimately the plugged nozzle will fail to fire and no image will be generated.
Several methods of addressing the crusting problems are known in the art. For example, most ink jet printers are designed to prevent excessive evaporation of solvent from pen nozzles by seating the pen cartridge in an air tight chamber when not in use. These devices become ineffective with continued printer use because deposits form at the rubber seals and the system loses its airtight condition. Another known method is to employ a wiper that removes solids formed at the surface of the nozzle. This device is often ineffective because the depth or hardness of the plug resists mechanical removal. It is also known to use forced air or vacuum suction to clear the nozzle. These methods are often inefficient and add considerable expense to the cost of the printer.
A commonly used method to clear the plug is to fire the pen in a non-image mode, i.e. into a collection receptacle or "spittoon". While this solution is the most effective remedy, it requires that the ink form a "soft" plug, i.e., one which is mechanically or cohesively weak.
Other attempts have been made to solve the decap problem by adding hygroscopic agents which reduce the rate of water evaporation by their ability to pick up water vapor from the air. While some improvement has been realized with this approach, a total solution to this problem has not been achieved. An alternative method of using a polymer blend of polyvinylpyrrolidone/polyvinylacetate copolymers and polyvinylpyrrolidone/poly(dimethylaminoethyl methacrylate) copolymers to solve the problem has been disclosed by Bearss et al in U.S. Pat. No. 4,791,165. The polymer blend forms a soft plug in the orifice of the ink jet pen and prevents further evaporation of the volatile components in the aqueous carrier. The soft plug is readily blown out upon ejection of ink droplets. However, a disadvantage of this method is that the use of the positively charged polymers tends to precipitate the anionic dyes which are most commonly used in aqueous ink jet inks.